REPORT ON PHYSICAL OPTICS. 373 



When Newton's rings are formed between glass and metal, — 

 the incident light being polarized, and the angle of incidence 

 exceeduig the polarizing angle of the glass, — it is found that the 

 rings dilate, as the azimuth of the plane of polarization with 

 respect to the plane of reflexion is increased; the dilatation 

 being a maximum when these two planes become perpendicular. 

 In order to account for this fact. Professor Airy has shown, that 

 if the vibrations of the incident pencil be resolved into two, one 

 in the plane of incidence, and the other in the perpendicular 

 plane, it is necessary to assume that their phases are unequally 

 changed by rejiexion ; the phases of the vibrations in the plane 

 of reflexion being more retarded than in the perpendicular plane. 

 The two oppositely polarized portions, therefore, will differ in 

 phase after reflexion, and will therefore compound a pencil ellip- 

 tically -polarized. Professor Airy has observed a similar pheno- 

 menon when Newton's rings were formed between diamond and 

 plate-glass, the angle of incidence being a few degrees less than 

 the maximum polarizing angle of diamond ; and he concludes 

 \h'dX, for such incidences, the nature of reflexion from diamond is 

 analogous to metallic reflexion. 



Sir David Brewster has extended his researches on the subject 

 of metallic reflexion to a great variety of cases, and has traced the 

 eifects of successive reflexions in the same, or in different planes ; 

 and at the same, or different angles. When the light which has 

 been restored to plane-polarized light, by two reflexions in the 

 same plane and at the maximum polarizing angle, undergoes a 

 third reflexion under the same circumstances, it becomes again 

 elliptically-polarized. By a fourth reflexion it is again restored to 

 plane-polarized light, the plane of polarization being, however, 

 brought nearer to the plane of reflexion. This continued ap- 

 proach of the plane of polarization to the plane of reflexion, 

 enables the author to explain, according to his peculiar views, 

 the effect of successive reflexions upon common light. 



It remains, further, to extend the theory of Fresnel to reflexion 

 at the surface of a medium in which the elasticity of the ether 

 is different in different directions. All that we know on this 

 interesting subject we owe to the unwearied zeal of Sir David 

 Brewster. It had been supposed by Malus, and the opinion 

 seems to have passed current with succeeding philosophers, that 

 the exterior surfaces of crystallized substances acted upon the 

 reflected light exactly in the same manner as the surfaces of or- 

 dinary media ; or, in the language of the theory of emission, 

 that the reflecting forces extended beyond the limits of the ^;o- 

 larizing forces oiihe cvj^^^. Sir David Brewster was led to 

 doubt this opinion; and in the year 1819 he undertook an ex- 



